1. Field of the Invention
The present invention relates to a method of and apparatus for repairing high velocity fluid flow machinery such as turbines and in particular turbine blades.
Turbine blades are subjected to wear during use and, particularly in the case of steam turbines, it is the leading edge of the outer ends of the turbine blades in a rotor assembly that is most prone to erosion.
Erosion generally occurs because of contact between the blades and particles carried by the steam and also to a certain extent with the steam itself.
In view of the erosion that occurs, particularly at the leading outer edge of a turbine blade, it is usual to make that part of the blade as an insert made from suitable hardened steel or from another hard material, such as Stellite, or to provide the blade with a covering of such material, such piece or covering being secured to the blade by welding or brazing for example.
Inevitably, after a period of use, the turbine blades have to be replaced or repaired due to erosion that has occurred and possible other damage such as cracks or foreign object damage. In view of the high cost of replacing blades, it is desirable that a satisfactory and long lasting repair can be carried out and that such repairs can be carried out as quickly as possible so as to minimise down time of the turbine.
In the case of repairs such as, for example, the replacement of a hard metal shield or insert, considerable thermal energy is imparted to the blade during a welding or brazing operation which in some instances can cause considerable distortion to the blade.
2. Prior Art
It has been proposed in British patent specification 2,124,126 to counter said distortion by deliberately heating the blade in a manner to cause distortion in a direction opposite to the distortion that is expected to occur during such a welding or brazing operation.
It is sometimes desirable to carry out repairs on turbine blades in situ, that is when the blades are still attached to a "wheel" on a turbine shaft, and in such cases it is desirable to pre-distort the blade prior to the operation of repair requiring the input of considerable thermal energy. However, since the blade is still attached to the wheel, considerable problems arise in controlling the distortion input to the correct part of the blade.